docs: def merge(left, right):
      """
      deep merge dictionary on the left with the one
      on the right.
  
      Fill in left dictionary with right one where
      the value of the key from the right one in
      the left one is missing or None.
      """
      if isinstance(left, dict) and isinstance(right, dict):
          for key, value in right.items():
              if key not in left:
                  left[key] = value
              elif left[key] is None:
                  left[key] = value
              else:
                  left[key] = merge(left[key], value)
      return left